select() and pselect() allow a program to monitor multiple file
descriptors, waiting until one or more of the file descriptors become
"ready" for some class of I/O operation (e.g., input possible). A
file descriptor is considered ready if it is possible to perform a
corresponding I/O operation (e.g., read(2), or a sufficiently small
write(2)) without blocking.
select() can monitor only file descriptors numbers that are less than
FD_SETSIZE; poll(2) does not have this limitation. See BUGS.
The operation of select() and pselect() is identical, other than
these three differences:
(i) select() uses a timeout that is a struct timeval (with seconds
and microseconds), while pselect() uses a struct timespec
(with seconds and nanoseconds).
(ii) select() may update the timeout argument to indicate how much
time was left. pselect() does not change this argument.
(iii) select() has no sigmask argument, and behaves as pselect()
called with NULL sigmask.
Three independent sets of file descriptors are watched. The file
descriptors listed in readfds will be watched to see if characters
become available for reading (more precisely, to see if a read will
not block; in particular, a file descriptor is also ready on end-of-
file). The file descriptors in writefds will be watched to see if
space is available for write (though a large write may still block).
The file descriptors in exceptfds will be watched for exceptional
conditions. (For examples of some exceptional conditions, see the
discussion of POLLPRI in poll(2).)
On exit, each of the file descriptor sets is modified in place to
indicate which file descriptors actually changed status. (Thus, if
using select() within a loop, the sets must be reinitialized before
each call.)
Each of the three file descriptor sets may be specified as NULL if no
file descriptors are to be watched for the corresponding class of
events.
Four macros are provided to manipulate the sets. FD_ZERO() clears a
set. FD_SET() and FD_CLR() add and remove a given file descriptor
from a set. FD_ISSET() tests to see if a file descriptor is part of
the set; this is useful after select() returns.
nfds should be set to the highest-numbered file descriptor in any of
the three sets, plus 1. The indicated file descriptors in each set
are checked, up to this limit (but see BUGS).
The timeout argument specifies the interval that select() should
block waiting for a file descriptor to become ready. The call will
block until either:
* a file descriptor becomes ready;
* the call is interrupted by a signal handler; or
* the timeout expires.
Note that the timeout interval will be rounded up to the system clock
granularity, and kernel scheduling delays mean that the blocking
interval may overrun by a small amount. If both fields of the
timeval structure are zero, then select() returns immediately. (This
is useful for polling.) If timeout is NULL (no timeout), select()
can block indefinitely.
sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is
not NULL, then pselect() first replaces the current signal mask by
the one pointed to by sigmask, then does the "select" function, and
then restores the original signal mask.
Other than the difference in the precision of the timeout argument,
the following pselect() call:
ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
is equivalent to atomically executing the following calls:
sigset_t origmask;
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
pthread_sigmask(SIG_SETMASK, &origmask, NULL);
The reason that pselect() is needed is that if one wants to wait for
either a signal or for a file descriptor to become ready, then an
atomic test is needed to prevent race conditions. (Suppose the sig‐
nal handler sets a global flag and returns. Then a test of this
global flag followed by a call of select() could hang indefinitely if
the signal arrived just after the test but just before the call. By
contrast, pselect() allows one to first block signals, handle the
signals that have come in, then call pselect() with the desired sig‐mask, avoiding the race.)
The timeout
The time structures involved are defined in <sys/time.h> and look
like
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
and
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
(However, see below on the POSIX.1 versions.)
Some code calls select() with all three sets empty, nfds zero, and a
non-NULL timeout as a fairly portable way to sleep with subsecond
precision.
On Linux, select() modifies timeout to reflect the amount of time not
slept; most other implementations do not do this. (POSIX.1 permits
either behavior.) This causes problems both when Linux code which
reads timeout is ported to other operating systems, and when code is
ported to Linux that reuses a struct timeval for multiple select()s
in a loop without reinitializing it. Consider timeout to be unde‐
fined after select() returns.

On success, select() and pselect() return the number of file
descriptors contained in the three returned descriptor sets (that is,
the total number of bits that are set in readfds, writefds,
exceptfds) which may be zero if the timeout expires before anything
interesting happens. On error, -1 is returned, and errno is set to
indicate the error; the file descriptor sets are unmodified, and
timeout becomes undefined.

EBADF An invalid file descriptor was given in one of the sets.
(Perhaps a file descriptor that was already closed, or one on
which an error has occurred.) However, see BUGS.
EINTR A signal was caught; see signal(7).
EINVAL nfds is negative or exceeds the RLIMIT_NOFILE resource limit
(see getrlimit(2)).
EINVAL The value contained within timeout is invalid.
ENOMEM Unable to allocate memory for internal tables.

select() conforms to POSIX.1-2001, POSIX.1-2008, and 4.4BSD (select()
first appeared in 4.2BSD). Generally portable to/from non-BSD
systems supporting clones of the BSD socket layer (including System V
variants). However, note that the System V variant typically sets
the timeout variable before exit, but the BSD variant does not.
pselect() is defined in POSIX.1g, and in POSIX.1-2001 and
POSIX.1-2008.

An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET()
with a value of fd that is negative or is equal to or larger than
FD_SETSIZE will result in undefined behavior. Moreover, POSIX
requires fd to be a valid file descriptor.
The operation of select() and pselect() is not affected by the
O_NONBLOCK flag.
On some other UNIX systems, select() can fail with the error EAGAIN
if the system fails to allocate kernel-internal resources, rather
than ENOMEM as Linux does. POSIX specifies this error for poll(2),
but not for select(). Portable programs may wish to check for EAGAIN
and loop, just as with EINTR.
On systems that lack pselect(), reliable (and more portable) signal
trapping can be achieved using the self-pipe trick. In this
technique, a signal handler writes a byte to a pipe whose other end
is monitored by select() in the main program. (To avoid possibly
blocking when writing to a pipe that may be full or reading from a
pipe that may be empty, nonblocking I/O is used when reading from and
writing to the pipe.)
Concerning the types involved, the classical situation is that the
two fields of a timeval structure are typed as long (as shown above),
and the structure is defined in <sys/time.h>. The POSIX.1 situation
is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
where the structure is defined in <sys/select.h> and the data types
time_t and suseconds_t are defined in <sys/types.h>.
Concerning prototypes, the classical situation is that one should
include <time.h> for select(). The POSIX.1 situation is that one
should include <sys/select.h> for select() and pselect().
Under glibc 2.0, <sys/select.h> gives the wrong prototype for pse‐lect(). Under glibc 2.1 to 2.2.1, it gives pselect() when
_GNU_SOURCE is defined. Since glibc 2.2.2, the requirements are as
shown in the SYNOPSIS.
Correspondence between select() and poll() notifications
Within the Linux kernel source, we find the following definitions
which show the correspondence between the readable, writable, and
exceptional condition notifications of select() and the event notifi‐
cations provided by poll(2) (and epoll(7)):
#define POLLIN_SET (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP |
POLLERR)
/* Ready for reading */
#define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
/* Ready for writing */
#define POLLEX_SET (POLLPRI)
/* Exceptional condition */
Multithreaded applications
If a file descriptor being monitored by select() is closed in another
thread, the result is unspecified. On some UNIX systems, select()
unblocks and returns, with an indication that the file descriptor is
ready (a subsequent I/O operation will likely fail with an error,
unless another process reopens file descriptor between the time
select() returned and the I/O operation is performed). On Linux (and
some other systems), closing the file descriptor in another thread
has no effect on select(). In summary, any application that relies
on a particular behavior in this scenario must be considered buggy.
C library/kernel differences
The Linux kernel allows file descriptor sets of arbitrary size,
determining the length of the sets to be checked from the value of
nfds. However, in the glibc implementation, the fd_set type is fixed
in size. See also BUGS.
The pselect() interface described in this page is implemented by
glibc. The underlying Linux system call is named pselect6(). This
system call has somewhat different behavior from the glibc wrapper
function.
The Linux pselect6() system call modifies its timeout argument. How‐
ever, the glibc wrapper function hides this behavior by using a local
variable for the timeout argument that is passed to the system call.
Thus, the glibc pselect() function does not modify its timeout argu‐
ment; this is the behavior required by POSIX.1-2001.
The final argument of the pselect6() system call is not a sigset_t *
pointer, but is instead a structure of the form:
struct {
const kernel_sigset_t *ss; /* Pointer to signal set */
size_t ss_len; /* Size (in bytes) of object
pointed to by 'ss' */
};
This allows the system call to obtain both a pointer to the signal
set and its size, while allowing for the fact that most architectures
support a maximum of 6 arguments to a system call. See
sigprocmask(2) for a discussion of the difference between the kernel
and libc notion of the signal set.

POSIX allows an implementation to define an upper limit, advertised
via the constant FD_SETSIZE, on the range of file descriptors that
can be specified in a file descriptor set. The Linux kernel imposes
no fixed limit, but the glibc implementation makes fd_set a fixed-
size type, with FD_SETSIZE defined as 1024, and the FD_*() macros
operating according to that limit. To monitor file descriptors
greater than 1023, use poll(2) instead.
The implementation of the fd_set arguments as value-result arguments
means that they must be reinitialized on each call to select(). This
design error is avoided by poll(2), which uses separate structure
fields for the input and output of the call.
According to POSIX, select() should check all specified file
descriptors in the three file descriptor sets, up to the limit
nfds-1. However, the current implementation ignores any file
descriptor in these sets that is greater than the maximum file
descriptor number that the process currently has open. According to
POSIX, any such file descriptor that is specified in one of the sets
should result in the error EBADF.
Glibc 2.0 provided a version of pselect() that did not take a sigmask
argument.
Starting with version 2.1, glibc provided an emulation of pselect()
that was implemented using sigprocmask(2) and select(). This
implementation remained vulnerable to the very race condition that
pselect() was designed to prevent. Modern versions of glibc use the
(race-free) pselect() system call on kernels where it is provided.
Under Linux, select() may report a socket file descriptor as "ready
for reading", while nevertheless a subsequent read blocks. This
could for example happen when data has arrived but upon examination
has wrong checksum and is discarded. There may be other
circumstances in which a file descriptor is spuriously reported as
ready. Thus it may be safer to use O_NONBLOCK on sockets that should
not block.
On Linux, select() also modifies timeout if the call is interrupted
by a signal handler (i.e., the EINTR error return). This is not
permitted by POSIX.1. The Linux pselect() system call has the same
behavior, but the glibc wrapper hides this behavior by internally
copying the timeout to a local variable and passing that variable to
the system call.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 SELECT(2)